Re: [PATCH tip/core/rcu 4/6] rcu: Silence compiler array out-of-bounds false positive

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Jan 07, 2013 at 05:18:37PM -0500, Steven Rostedt wrote:
> On Mon, 2013-01-07 at 09:19 -0800, Paul E. McKenney wrote:
> > On Mon, Jan 07, 2013 at 09:16:02AM -0800, Paul E. McKenney wrote:
> > > On Mon, Jan 07, 2013 at 07:50:02AM -0800, Josh Triplett wrote:
> > > > On Sat, Jan 05, 2013 at 09:09:36AM -0800, Paul E. McKenney wrote:
> > > > > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > > > > 
> > > > > It turns out that gcc 4.8 warns on array indexes being out of bounds
> > > > > unless it can prove otherwise.  It gives this warning on some RCU
> > > > > initialization code.  Because this is far from any fastpath, add
> > > > > an explicit check for array bounds and panic if so.  This gives the
> > > > > compiler enough information to figure out that the array index is never
> > > > > out of bounds.
> > > > > 
> > > > > However, if a similar false positive occurs on a fastpath, it will
> > > > > probably be necessary to tell the compiler to keep its array-index
> > > > > anxieties to itself.  ;-)
> > > > > 
> > > > > Markus Trippelsdorf <markus@trippelsdorf.de>
> > > > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > > > ---
> > > > >  kernel/rcutree.c |    4 ++++
> > > > >  1 files changed, 4 insertions(+), 0 deletions(-)
> > > > > 
> > > > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > > > > index d145796..e0d9815 100644
> > > > > --- a/kernel/rcutree.c
> > > > > +++ b/kernel/rcutree.c
> > > > > @@ -2938,6 +2938,10 @@ static void __init rcu_init_one(struct rcu_state *rsp,
> > > > >  
> > > > >  	BUILD_BUG_ON(MAX_RCU_LVLS > ARRAY_SIZE(buf));  /* Fix buf[] init! */
> > > > >  
> > > > > +	/* Silence gcc 4.8 warning about array index out of range. */
> > > > > +	if (rcu_num_lvls > RCU_NUM_LVLS)
> > > > > +		panic("rcu_init_one: rcu_num_lvls overflow");
> > > > 
> > > > Why not write this as BUG_ON(rcu_num_lvls > RCU_NUM_LVLS)?  Given that
> > > > the condition in question can never happen, you don't really need an
> > > > explanatory message.
> > > 
> > > Good point, will do!
> > 
> > Ah, wait, BUG_ON() sometimes compiles to nothingness:
> > 
> > #ifndef HAVE_ARCH_BUG_ON
> > #define BUG_ON(condition) do { if (condition) ; } while(0)
> > #endif
> > 
> > So I do need the explicit "if".  :-(
> 
> Bah, those archs shouldn't be bothered with. If they don't want to bug,
> then that's there problem :-)

;-) ;-) ;-)

> Lots of places in the kernel have BUG_ON() where they require it to
> panic.

Fair point, but that doesn't mean that I want them complaining to
me when as a result of the compiler's array-index anxieties.

							Thanx, Paul